Prior art panoramic dental X-ray apparatus are well known in the art. Some provide a continuous image of the dental arch area and commonly employ an X-ray source and X-ray film supported on a rotatable carrying arm which orbits a patient situated in the beam path. The patient may remain stationary in the chair, or the chair may be transported in accordance with various X-Y type drive mechanisms in order to simulate the generally elliptical shape of the human dental arch, as disclosed in U.S. Pat. No. 4,125,774, assigned to the present assignee. The continuous image radiograph provides the dentist with a panoramic view of the teeth and associated structures and is a useful diagnostic aid in many phases of dental practice.
Various other prior art apparatus provide a discontinuous, or split image panoramic radiograph which possesses certain advantages. Here, the dentist is provided with additional interpretive information since two distinctly different views of the incisors, or centrals area are provided. For example, in U.S. Pat. No. 3,045,118, apparatus is disclosed which automatically shifts the patient in order that the line of sight between the X-ray source and film bypasses the patient's spinal column and permits X-raying of the other half of the dental arch. Apparatus is also disclosed therein for continuously moving an X-ray source and extra-oral film holder about the patient.
In U.S. Pat. No. 2,798,958, means are disclosed for reorienting the patient after completion of one-half of the excursion cycle in order to relocate the center of the axis of rotation with respect to the patient's head prior to X-raying the other one-half of the dental arch in order to provide the discontinuous, or split radiographic images.
Regardless of the type radiographic image to be obtained, i.e., continuous or discontinuous, compensation is usually made for the fact that the curvature of the desired area of focus is generally not a true circle or ellipse. Thus, the rate of film travel must be varied in accordance with the chair shift or transport, and the rate of travel of the X-ray source about the patient's head in order that the radiological projections occupy a distance on the film equal to the linear distance of a curved structure being X-rayed, such as a typical dental arch.
The present invention discloses film drive mechanism which is readily adaptable for use with structure disclosed in cross-referenced patent application Ser. No. 002,148, for "Panoramic Dental X-Ray Machine X-Motion Drive," or with other suitable panoramic dental X-ray machines which provide either or both continuous and discontinuous radiographic images.